I've seen lot of Hollywood horror movies, and most of the directors using a mirror as a ghost's sign or ghosts using mirror as a medium to talk with humans. I couldn't understand what would be the reason for that.

@stevvve,There are many movies,for instance Candyman.In that movie, Candyman has summoned by anyone who looks into a mirror.
– Vijin PaulrajApr 1 '12 at 17:39

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In addition to what is pointed out in the answers, it makes for an instant creep-out moment, when the mirror image acts independently of the person in the "real" world.
– PoloHoleSetAug 15 '16 at 20:50

3 Answers
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It's worth noting that mirrors (and any other reflective material for that matter) are a well-used narrative tool for presenting the duality of characters on screen, whether they are protagonists or antagonists (or, more likely, somewhere in between).

As DForck42 has already pointed out, there are many urban legends and stories connected to mirrors as well as popular tales such as Alice Through the Looking-Glass which suggest an alternate world that is at once recognizable yet unfamiliar and dangerous.

The notions of souls being trapped in mirrors, or of mirrors being portals to another world, are as old as time - and therefore a powerful element to be exploited in ghost/horror movies.

As the other answers pointed out, there are a lot of urban legends related to mirrors and reflections. And also, whenever we look in a mirror, we see ourselves, but everything is inverted (Horizontally). So horror movies may employ this observation to show that we (or someone) is seeing a laterally inverted personality or character (for example, it may symbolize the good person looking at the mirror and seeing the "bad" version of himself).

And I think also that there's a common [superstitious?] saying that many ghosts are seen only through mirrors and camera lenses. That may explain why [in movies], some persons look in the mirror and instead of seeing themselves, they see a ghost of themselves (or a ghost standing besides them)- a previous life of themselves or an exactly similar situation that happened before, but which ended in tragic circumstances (the mirrors have "trapped" the situation).

Also on a more logical note, from ancient times, mirrors have played a popular role of "demon doors". The reason for that may be that in ancient times, reflections are so supernatural-looking to ancient people and what they may have seen would be an alternate reality. Hence treating mirrors as gateways to demons and gods.